The answer to this question will come as a surprise for many, and before answering I would like to clarify a couple variables.
I would like to categorize two sizes of Discus fish, an adult 6-inch and juvenile 3 inch, also that the Discus fish which stopped eating were healthy and eating before they began their hunger strike.
Discus fish which stop eating I have noticed will start to shut down in order to burn fewer calories. They do not move much and at the latter stages become lethargic and even disoriented. In the wild, during the dry season Discus fish can get trapped in small pools with no flowing water or a continuous supply of food. In order to survive these lean times, they go into this shut down mode to survive. My observations have been, when leaving Discus fish for a week un-fed, due to me being absents or out of town. When I return and start feeding again it takes a couple days for them to get their appetite back to where it was before I stopped feeding. Upon my return I give them their first meal for the week, they aggressively go after the food as if they are hungry, but don’t eat very much and most of the food sits on the bottom of the tank. This normally lasts for a couple days before their appetite and consumption returns to normal.
If an adult Discus fish stopped eating it would take 9 months to a year before it finally starved to death, and a 3-inch fish would take 3 to 5 months before its demise from starvation. With this length of time not eating, Discus fish will lose body mass, their belly will appear skinny or caved in. Once Discus fish loses body mass above the lateral line, giving the Discus a pinched in appearance along its spine, this will be the point of no return and the fish will succumb to death from starvation. If your Discus fish stopped eating and died quickly, or within weeks after they last ate, this was not due to starvation, but instead something else which killed the Discus fish and you need to check other aquarium parameter to figure out a cause of death.
It is important when you feed Discus fish to be sure they all aggressively attack the food. Watch for stragglers that hold back and don’t feed with the rest. If you notice these symptoms take immediate steps to correct these beginning signs of appetite loss from worsening. The most effective remedy is to turn the heat up to 90 degrees at these first signs, to stimulate and regain appetite. At this stage the outcome to save the Discus fish will be at its best. The longer left un-treated the harder it will be to cure, requiring higher heat to stimulate appetite and getting your Discus fish eating again. Discus fish which have gone off food for weeks will need more drastic treatment and their temperature increased to 94 degrees until they take interest in food again. So, the quicker you notice them not eating, the more effective and less drastic treatment has to be for a positive outcome.
Read More: How Long Do Discus Fish Live